Ink-jet printers have been widely used for printing on papers, films, cloths, etc. not only at offices but also at homes along with popularization of computers in recent years.
Methods for ink-jet recording include methods of applying pressure to an ink by a piezo device to discharge droplets; methods of generating bubbles in an ink by heating the ink to discharge droplets; methods using a ultrasonic wave; and methods of aspirating and discharging droplets by an electrostatic force. Ink compositions used in the ink-jet recording methods are water inks, oil-based inks and solid (melting-type) inks. Among the inks, the water inks have mainly been used from the viewpoints of productivity, handling, odor, safeness, etc.
Coloring agents contained in the inks for ink-jet recording are required to have a high solubility in a solvent; to be capable of recording with a high density; to be excellent in a hue; to be excellent in fastnesses to lights, heat, air, water, chemicals, etc.; to hardly blur on an image-receiving material with excellent fixation; to be excellent in storage stability as an ink; to have no toxicity; to be high in purity; and to be available at a low price. However, it is extremely difficult to find a coloring agent satisfying these requirements at high levels.
Although various dyes and pigments have been proposed and put into practical use as a coloring agent for ink-jet recording, a coloring agent satisfying all the requirements have not been obtained yet at present. It is difficult for conventionally well-known dyes and pigments such as those having a Color Index (C.I.) number to have the hue and fastnesses sufficient for ink-jet recording.
Food black dyes, naphthol-based direct azo dyes, acidic azo dyes, etc. have been widely known as black dyes for ink-jet recording.
The food black dyes include C.I. Food Black 1 and C.I. Food Black 2 as typical examples, and techniques for using them in black inks for ink-jet recording are described in JP-A-2-36276 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”), JP-A-2-233782 and JP-A-2-233783, etc.
Techniques for using the acidic azo dyes such as C.I. Acid Blacks 2, 31, 52, 140 and 187 in black inks for ink-jet recording are described in JP-A-60-108481, JP-A-2-36277 and JP-A-2-36278, etc. Further, techniques for using the direct azo dyes such as C.I. Direct Blacks 9, 17, 38, 51, 60, 102, 107, 122, 142, 154 and 168 in black inks for ink-jet recording are described in JP-A-56-139568, JP-A-61-285275 and JP-A-3-106974, etc.
In general, it is difficult to complete a black hue by one black dye, and whereby the black dye is preferably used in combination with a short wavelength dye.
Known short wavelength dyes are also direct azo dyes, acidic azo dyes, etc. Techniques for using C.I. Acid Yellows 17, 23, 49 and 194, C.I. Direct Yellows 86, 120, 132 and 144, etc. in black inks for ink-jet recording are described in JP-A-7-97541, WO 97/16496, JP-A-10-158560 and JP-A-11-12514, etc.
Dyes used in the present invention are superior to the above dyes in the fastnesses, and a technique for using them in black inks for ink-jet recording is described in Japanese Patent Application No. 2002-113460.
The inventors have investigated the inks comprising dyes for ink-jet. However, it became clear that water black inks had a disadvantage of low durability of image. Further, it was found that few black dyes could singly provide a black image with excellent quality of printed character, differently from carbon black, and the black dyes needed to be used as a mixture of several dyes having different absorption spectra. In this case, the quality of the black-containing image was remarkably deteriorated, when the dyes in the mixture had different fading rates, or when an absorption waveform of a dye with a broad absorption was changed by fading. The inventors have made an intense investigation of increasing the fastnesses of the dye. However, it became clear that the fastnesses of the dye were reduced when the ink composition contained a high concentration of metal ions.